The invention relates to a thread-cutting apparatus with reversal of the direction of rotation with a sun-and-planet gear with an axially displaceable reversing device for reversing the direction of rotation of a drill, held in the grip of a spindle, which can be pulled out preferably against a length equalization spring after a given drilling depth has been reached, especially reversing chucks with pressure equalization pipelines, which discharge into an equalization chamber, sealed by a movable wall from the outside air.
Previously, the construction of such a thread-cutting apparatus was such that a screw tap brought about the switching motion during the reversal. When the end depth of the thread is reached, the chuck is retracted over the controlled forward feed unit. At the same time, the drill is pulled out and this pulling-out path is the switching path for the transmission. Moreover, this pulling-out path represents a machine path, which is required for the free travel of the drill, but otherwise is unnecessary. This causes an air cutting of about 6 to 10 mm before the workpiece is reached and lengthens the tapping cycle and, with that, reduces the effectiveness of the manufacturing process as a whole.
The attainable accuracy of thread depth is affected owing to the fact that the NC machine switches at the exact depth dimension; however, as a result of the unfavorable position of tooth upon tooth of the sun-and-planet gear, the switching element may not be able to engage immediately. This inaccuracy, moreover, depends on the hardness of the length equalization spring, which connects the reversing device with the driven shaft of the chuck. A spring of greater hardness increases the attainable thread depth accuracy but, on the other hand, is responsible for the fact that, particularly in the case of smaller drills, the therefrom resulting high restoring force leads to increased flank friction at the screw tap and, as a result, to a loss of service life with the danger of damage to the thread.
It is therefore an object of the invention to devise a thread-cutting apparatus of the initially mentioned type in such a manner that, on the one hand, a high thread depth accuracy is achieved and, on the other, overloading the drill is avoided even in the case of small drill sizes.
To accomplish this objective pursuant to the invention, provisions are made so that the reversing device can be actuated by an external driving element, which can be actuated by a reversing signal, which is generated by an actuator.
Pursuant to the invention, the reversing no longer is initiated by pulling out the drill while moving back the controlled forward feed unit. Instead, it is initiated by a separate driving element, which also does not obtain its energy from the pulling out motion itself and instead is driven pneumatically or electrically, for example. The external driving may preferably be a 2-chamber hydraulic cylinder, which is actuated either by compressed air or by a coolant under pressure. Optionally, the driving element may also be a 1-chamber hydraulic cylinder which, making do with only one triggering line, works against a restoring spring. In a development of the invention, it is also possible to construct the external driving element as an electromagnetically driven actuator.
This relationship of the reversing device as regards the pull-out motion of the drill, not only makes it possible to design the length equalization spring to be very soft so that overloading of the drill with the danger of damaging the drill or the thread itself is avoided, but also, beyond this, makes possible a construction, for which the length equalization spring is omitted completely and a chuck can be used for synchronous thread cutting. The possibility of a softer design of the length equalization spring moreover furthermore also has the additional advantage, that the accuracy of the switching point is increased.
In accordance with a further distinguishing feature of the invention, the reversing signal can be generated either by a cam or the like at the grip or generated directly by the advance control which, together with the signal for the moving back of the forward feed unit, simultaneously can generate the reversing signal for the external driving element of the reversing device.
Further advantages, distinguishing features and details of the invention arise out of the following description of an example, as well as out of the accompanying drawing.